Every single message updates a TSThread model, but only a change in a
member's profile name or phone number, or a change to a group's model,
can update the indexing information. Turn 'shouldBeIndexedForFTS' into
a tri-state 'FTSIndexMode' with options 'never', 'manualUpdates', and
'always', and use 'manualUpdates' for TSThreads. Then explicitly
reindex on any of the changes listed above.
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